Inter-relationships between the geometrical parameters of the amino group in several para-substituted anilines M. Alcolea Palafox a, * , F.J. Mele ´ndez b a Departamento de Quı ´mica-Fı ´sica I (Espectroscopia), Facultad de Ciencias Quı ´micas, Universidad Complutense, Madrid-28040-ES, Spain b Lab. de Co ´mputo Cientı ´fico, Facultad de Ciencias de la Computacion, Beneme ´rita Universidad Auto ´noma de Puebla, Edif. 135. Cd. Universitaria, Puebla, Pue-72570, Mexico Received 23 September 1998; received in revised form 23 March 1999; accepted 2 July 1999 Abstract Several theoretical methods were used to obtain the relationships between the geometric parameters of the amino group in para-substituted anilines. Several equations were established. These relations were found in the majority of the cases as linear or slightly parabolic with a correlation coefficient close to the value one. The study was extended to the stretching frequencies of the amino group. Some general conclusions were drawn.  1999 Elsevier Science B.V. All rights reserved. Keywords: Substituted anilines; Semiempirical methods; Ab initio methods 1. Introduction The para-substituted anilines are important mole- cules for chemical, phamacological and biological processes. Some of their derivatives are local anes- thetics, and in these molecules the amino group plays an important role in the interactions with the receptor. However, its exact geometry is unknown, especially the inversion v and tilt angles e , Fig. 1. Experimental data for the inversion angle have been reported in detail only in the aniline molecule and refer to the g angle. Some of the values have been 37:48 ^ 2° [1] and 42:4 ^ 0:3° [2], obtained by micro- wave, 38 ^ 3° by X-ray [3], 42° by resonance fluor- escence measurements and vibrational analysis of far IR spectra [4,5], 41.7° by high-resolution electronic spectroscopy [6] and 38° by far infrared spectroscopy [7]. Values of g have also been determined in p- diaminobenzene, 43–46° by X-ray [8] and 43 ^ 4° by electron diffraction [9], and in p-fluoroaniline, 41.2° by far IR spectroscopy [6]. The inversion angle v is normally used as a measure of the pyramidalization of the amino group. This deformation from planarity of the amine nitrogen has been interpreted [10,11] as resulting from a balance between opposing forces: the stability gained by the molecule as a whole arising from p– p conju- gation of the nitrogen lone pair with the aromatic system versus that gained by the amine using highly directed sp 3 orbitals for bond formation. This asym- metric interaction between the amino group and the benzene ring [12] produces a small displacement of the nitrogen atom out of the benzene ring plane and the small tilt angle e , Fig. 1. For a better knowledge of this amino group, the present paper establishes structural similarities among the values of ten selected p-amino derivatives (Fig. 2). Their geometries were completely optimized using the semiempirical procedures AM1 [13] and SAM1 [14] implemented in the ampac5.0 program Journal of Molecular Structure (Theochem) 493 (1999) 171–177 0166-1280/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0166-1280(99)00237-7 www.elsevier.nl/locate/theochem * Corresponding author.